This article will explain the differences between counting days or nights. Count is used to specify if days or nights are used when calculating quantity for the 'Date Range Updates Quantity Setting'.This setting only applies when you are using the Product or General profiles.
You can locate the setting in the Date Range section of the product page just after the Duration settings.
Days- Counts per day that passes.
ie. If you book January 1st for 1 Day, the booking runs from January 1st to January 1st. (Morning of January 1st to evening of January 1st.)
Nights- Counts per night that passes.
ie. If you book January 1st for 1 Night, the booking runs from January 1st to January 2nd. (Evening of January 1st to morning of January 2nd).
Either scenario would still allow January 2nd to be available for booking as long as no lead or lag time has been applied.